1. Field of the Invention
Embodiments of the present invention relate, in general, to audio signal processing, and more particularly to modifying the amplitude of a digital audio signal so that perceived loudness is maintained at a consistent level.
2. Relevant Background
Audio levels across varying signal sources vary. One particularly annoying feature of having access to multiple channels on television, thousands of songs from hundreds of artists on a single MP3 player, and multiple applications on mobile phones is that just as quickly as one can change from one audio signal source to another, so too does the volume level change. Indeed, as a television viewer changes from one channel to the next, (or even when the programming changes on the existing channel), the volume the viewer set previously can vastly change with no viewer input.
FIG. 1 is a graphical depiction of four input signals from four different television channels. The amplitude of the signals of each channel 110, 120, 130, 140 varies significantly. Thus a person changing from the first television channel 110 to the second television channel 120 would experience a significant increase in volume. If the individual adjusted the television to a normal audio level, upon changing the set to the third television channel 130 the audio would likely soften. In this environment (in which audio signals change along with input signals) viewers must manually modify the volume setting upon each input change to maintain a reasonable audio level.
Many attempts have been made to tackle the issue of varying volume across different content presented via a particular media player, e.g. volume variances in television programs. A simple approach known to one of reasonable skill in the art is to employ a Dynamic Range Compressor (DRC) to suppress excessively loud signals and boost soft ones. A slightly more advanced approach would be to divide the signal into a number of sub-bands and apply a DRC to each sub-band independently. This technique is commonly referred to as multi-band DRC (MDRC). However, without analyzing the content of the signal but simply responding to the signal level according to a predefined output-input relationship curve, whether the response is based on amplitude or power, tends to distort the signal and produce unrealistic sounds. For instance, the attack or initial portion of a signal is attenuated while the release or trailing portion of a signal is amplified. The result is a less pleasant listening experience.
Television programming is very diverse. It ranges from classical music and opera to rock concerts, from romantic movies to disaster and horror films, and from news commentary to sporting events, just to name a few. And interspersed throughout each of these programs are advertisements designed to capture attention and evoke an emotional response. Television advertisements, generally speaking, are louder than a news broadcast or a drama/romantic movie. Instead, they are akin to a sporting event or a disaster movie, with respect to the volume in which they are presented.
It is commonplace for individuals watching television or using similar media presentation devices to manually adjust the volume each time a new piece of content is presented. When watching television, people often experience a considerable increase in volume upon the initiation of a commercial driving them to reduce the volume to a more pleasant level. Then upon resumption of their chosen content the volume has to be increased. Similarly when using a MP3 player, changing artists or shifting from one genre to another, one can experience dramatic volume changes. There is a long felt need for a system and/or methodology to eliminate this annoying experience by measuring and analyzing an audio signal so that a perceived loudness can be maintained at a substantially constant level. These and other challenges of the prior art are addressed by one or more embodiments of the present invention.